Characterization of cryogenic Fe-6Ni steel fracture modes: A three dimensional quantitative analysis
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I.
INTRODUCTION
THE development of new materials to meet innovative technological applications demands knowledge of the complex processes of fracture that take place under varied testing conditions. This is of particular importance in the development of materials for use at cryogenic temperatures where the ductile-to-brittle transition (DBT) may occur. Precise understanding of the fracture mechanisms is needed to predict and control the DBT for alloy design purposes. Fracture surfaces represent the final state of a rapid process. The sequence of events during fracture is microscopic and is difficult, if not impossible, to observe as it proceeds. In studying the mechanism(s) of fracture, we are therefore confined to varying the initial set of conditions, e.g., testing temperature, stress level, loading mode, etc., and observing the end result, i.e., fracture surfaces. Despite this limited information, past research has provided many interpretations and descriptions of fracture mechanisms. For DBT fracture mechanisms in particular, two different models have gained major support: 1. One model assumes that there is only one fracture mechanism operating in the transitional stage from ductile to brittle mode. This mechanism results in different fracture appearances at various testing temperatures because of the difference in the amount of plastic deformation that occurs before fracture is completed. 2. The other model, instead, assumes that there are two different fracture mechanisms, each being fully activated at opposite ends of the temperature scale. This dual mechanism model results in a mixture of two modes of fracture being simultaneously present at intermediate temperatures when each mechanism is only partially activated. One method of investigating fracture mechanisms is the analysis of the resulting fracture surfaces. Quantitative fractography is often used to characterize two dimensional (2D) observations of fracture surfaces made through optical or electron microscopy.~-8 The use of stereo imaging techG.O. FIOR, formerly with the Lawrence Berkeley Laboratory. University of California, Berkeley, CA 94720, is Process Development Manager with TEGAL Corp.. 11 Digital Dr., P.O. Box 10, Novato, CA 94948. J. W. MORRIS, Jr. is Professor of Metallurgy, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted June 12. 1984. METALLURGICAL TRANSACTIONS A
niques to obtain three dimensional (3D) qualitative and quantitative information on fracture surface, however, can effectively extend the perception of the real fracture appearance by revealing additional information about depth, roughness, curvature, etc., that conventional 2D images cannot provide. 9-12 In this work, a quantitative stereoscopic fractographic analysis of cryogenic steel is used to characterize the fracture modes of an Fe-6Ni alloy over the range of temperature where its DBT occurs.
II.
EXPERIMENTAL PROCEDURE
A. Material and Heat Treatment The material used for this study was a comme
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